This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to Patent Serial No. 0000688-2 filed in Sweden on Mar. 2, 2000, the entire content of which is hereby incorporated by reference.
1. Technical Area of the Invention
The present invention relates to a rock drill bit for percussive drilling, especially for top hammer drilling, and a method for the manufacturing of a rock drill bit.
2. Prior Art
A percussive drill bit and its associated drill string comprising drill rods shall be removed from a drilled hole after the hole is completed. During removal, the drill bit and the drill string are usually rotated in an opposite direction compared to the direction employed for drilling of the drilled hole. However, the removal of the drill bit and drill string from the drilled hole is impaired by loose rock material from the bore wall. To correct this problem it is customary that the drill bit has at its rear end, i.e., at the end surface facing away from a drill bit front surface, inserts or chisels, hereinafter called retrac teeth, which during rotation of the drill string and the drill bit, in connection with retraction of the drill string and the drill bit from the drilled hole, crush the rock material which has become loosened from the bore wall. The retrac teeth are made by means of special milling operations, which consequently constitutes additional manufacturing operations besides the usual operations for the manufacturing of a rock drill bit of the current type. However, the shaping of these known teeth at the rear end of the drill bit results in the formation of teeth having sharp corners. This promotes the generation of cracks in the teeth during operation of the drill bit.
U.S. Pat. No. 5,743,345 shows in FIG. 3 thereof a rock drill bit with an associated drill rod, where the drill bit is provided with special cemented carbide buttons at its rear end, which function in a corresponding manner to the above-described retrac teeth. The provision of special cemented carbide buttons at the rear end of the drill bit makes the manufacturing of a rock drill bit of the current type more expensive.
One object of the present invention is to provide a rock drill bit of the above-captioned type, wherein the retrac teeth of said rock drill bit are formed such that crack formation is avoided to a high degree in connection with said retrac teeth.
Another object of the present invention is to provide retrac teeth which permit the transfer of impact energy between the drill bit and the drill rod situated closest to the drill bit by means of so-called shoulder abutment.
The objects of the present invention are realized by a rock drill bit for percussive drilling comprising a head which defines a longitudinal center axis. The head includes an axially forward rock-crushing surface and a skirt extending axially rearwardly from the rock-crushing surface. An outer circumferential surface of the skirt is formed with circumferentially spaced apart grooves which form lands therebetween. The grooves and the lands extend at least generally in an axial direction. An axial rear portion of at least some of the lands form retrac teeth for crushing rock during withdrawal of the bit from a hole. Each retrac tooth includes a cutting edge extending along an outer circumference of the skirt.